The present invention relates to a method for heat treatment of silicon wafers, in particular, a method for heat treatment of silicon wafers that shows superior safety and can give silicon wafers of high quality.
As wafers for manufacturing semiconductor devices such as semiconductor integrated circuits, silicon wafers are mainly used. In such production of semiconductor devices, crystal defects which exist in wafer surface layers such as COPs (Crystal Originated Particles) can be mentioned as one of factors that degrade the yield. If such crystal defects exist in wafer surface layers, they may be a cause of degradation of electric characteristics of wafers. For example, in transistors of MOS structure, when high voltage is applied to a thermal oxide film formed on a wafer surface such as a gate oxide film, they may cause generation of dielectric breakdown of the oxide film.
As a further factor that worsens the yield of semiconductor device production, microroughness on wafer surfaces can be mentioned. It is known that microroughness that exists on wafer surfaces adversely affect carrier mobility directly under the gate oxide films (see Shinya Yamakawa, Hirai Ueno, Kenji Taniguchi, Chihiro Hamaguchi, Kazuo Miyatsuji, Umbert Ravaioli, J. Appl. Phys., 79, 911, 1995). In semiconductor devices, if degree of integration is increased, carrier mobility must correspondingly be increased. Moreover, with recent use of increasingly higher driving frequency of CPU, write time and read time of memories are required to be made higher. Therefore, it has come to be considered more important to make microroughness small in order to improve carrier mobility.
As a method for reducing crystal defects of silicon wafer surface layers, elimination of the defects by annealing heat treatment and so forth have been performed. A typical example thereof is high temperature hydrogen annealing. This method is a method for eliminating crystal defects by performing annealing heat treatment in a hydrogen atmosphere at a high temperature (see Japanese Patent Laid-open Publication (Kokai) No. 6-349839).
However, although a heat treatment in a hydrogen atmosphere can reduce crystal defects in wafer surface layers, it has a drawback that surfaces of wafers will be etched by the heat treatment. For example, when a heat treatment is performed at 1200xc2x0 C. for 60 minutes, about 0.5 xcexcm of silicon of wafer surface layers will be etched. For this reason, thickness of the portions of wafer surfaces with few crystal defects (defect-free layer) becomes small.
Furthermore, it is very dangerous to handle hydrogen gas at a high concentration at such a high temperature over a long period of time. Thus, it cannot be practically used without solving the problem of safety.
Therefore, there has also been proposed a method for eliminating crystal defects of wafer surface layers by performing a heat treatment using an inert gas such as argon for the atmosphere. However, although the crystal defects in wafer surface layers can be reduced by this method without etching wafer surface layers, it has a drawback that it worsens microroughness on wafer surfaces compared with that before the heat treatment.
In addition, there is also an abuse that local etching comes to be likely to occur due to the influence of a small amount of oxygen in the atmosphere, and thus haze is generated.
The term xe2x80x9chazexe2x80x9d used herein is an index of surface roughness, and means periodical waviness having a period of several to several tens of nanometers on the wafer surfaces. It is surface roughness that can be semi-quantitatively evaluated as a haze level of a whole wafer surface by scanning the whole wafer surface with a particle counter mainly utilizing a laser, and measuring strength of scattered reflection thereof.
Another method for avoiding the danger of hydrogen gas, there has also been contemplated a method utilizing a heat treatment with a hydrogen atmosphere and a heat treatment with an inert gas atmosphere such as argon in combination. This method comprises first performing a heat treatment of wafers in an inert gas atmosphere, and then performing a heat treatment of the wafers in a hydrogen atmosphere (see Japanese Patent Laid-open Publication No. 4-167433).
However, the heat treatment in an atmosphere containing hydrogen at the same temperature as the preceding heat treatment in an inert gas atmosphere will eventually etch wafer surfaces, and thus the defect-free layer thickness will become small.
Further, Japanese Patent Laid-open Publication No. 7-235507 discloses a method which comprises performing a heat treatment in an inert atmosphere, wherein hydrogen is introduced into the atmosphere during the temperature increasing or temperature decreasing period of the heat treatment. However, this method was accomplished with the purpose of preventing generation of slip dislocations in wafers by introducing hydrogen having heat conductivity higher than that of inert gas during the temperature increasing or temperature decreasing, and this method is not for eliminating crystal defects which exist in wafer surface layers or improving microroughness on wafer surfaces.
That is, this method simply comprises continuously introducing 1 liter/minute of hydrogen during the temperature increasing and decreasing, and an optimum composition of the heat treatment atmosphere during the temperature increasing and decreasing is unknown. Therefore, even if this method is used, the etching amount of the wafer surfaces may become large, or microroughness may be worsened. Thus, crystal defect density and surface roughness cannot be improved simultaneously.
As described above, among the conventional heat treatment methods, there are no method for reducing crystal defects of wafer surface layers without etching wafer surface layers and without degrading microroughness of wafers, with a little amount of hydrogen used. Therefore, it is desired to develop an effective method.
Furthermore, in the hydrogen annealing, the heat treatment is usually performed under a hydrogen gas atmosphere by increasing temperature at a temperature increasing rate of 1-10xc2x0 C./min, maintaining a temperature of from 950xc2x0 C. to the melting point of silicon for several hours, and then decreasing the temperature at a temperature decreasing rate of 2-5xc2x0 C./min (for example, Japanese Patent Publication (Kokoku) No. 5-18254 and Japanese Patent Laid-open Publication No. 6-295912). However, this heat treatment method has a drawback that the heat treatment requires a long period of time.
Therefore, it has been proposed a method for heat treatment using an apparatus for rapid heating and rapid cooling (Rapid Thermal Annealer, also abbreviated as xe2x80x9cRTA apparatusxe2x80x9d hereinafter) in order to shorten the heat treatment time etc. For example, in Japanese Patent Application No. 10-82606, the inventors of the present invention previously proposed a method for heat treatment of silicon wafers under a reducing atmosphere using an RTA apparatus, and proposed a method for heat treatment that can, in particular, reduce the COP density on surfaces of silicon wafers.
This method comprises a heat treatment of silicon wafers within a temperature range of from 1200xc2x0 C. to the melting point of silicon for 1-60 seconds under a reducing atmosphere. In this method, it is further preferred that 100% hydrogen or a mixed atmosphere of hydrogen and argon is used as the reducing atmosphere, and the heat treatment time is selected to be 1-30 seconds.
Further, it was found that COP density on surfaces of silicon wafers was markedly reduced and one of the electric characteristics, oxide dielectric breakdown voltage (Time Zero Dielectric Breakdown: TDDB), was also markedly improved by this method.
However, this method has a drawback that the aforementioned surface roughness on wafer surfaces after the heat treatment, called haze, may be degraded.
In addition, as mentioned above, it is known that surface roughness on wafer surfaces such as haze closely relates to performance and reliability of devices as a factor which greatly affects the electrical characteristics such as oxide dielectric breakdown voltage and carrier mobility (see Shinya Yamakawa et. al., J. Appl. Phys. 79, 911, 1996), and haze on wafer surfaces is considered a big problem.
Therefore, in Japanese Patent Application No. 10-176693, the inventors of the present invention proposed a method wherein the RTA heat treatment is performed with two or more divided steps as a method for solving that problem. In this method, a heat treatment of a preceding step is performed with the purpose of reduction of COPs, and a heat treatment of a subsequent step is performed in order to improve surface roughness on wafer surfaces such as haze.
Since this method can sufficiently improve haze while reducing COPs, it is a very useful method. However, because it requires performing two or more steps of high temperature heat treatments in an RTA apparatus, it has a drawback of complicated process steps. Such complicated process steps lead to increase of cost due to decrease of throughput. Therefore, it is desired to be further improved.
The present invention was accomplished in view of such problems as mentioned above, and an object of the present invention is to provide a method for heat treatment that can reduce crystal defects in wafer surface layers without etching wafer surface layers and without degrading microroughness on wafer surfaces with a small amount of hydrogen used.
Another object of the present invention is to provide a method for heat treatment of silicon wafers using a rapid heating and rapid cooling apparatus, which can reduce COPs and haze of wafer surfaces in a simpler manner.
In order to achieve the aforementioned objects, the present invention provides a method for heat treatment of silicon wafers, wherein a silicon wafer is subjected to a heat treatment at a temperature of from 1000xc2x0 C. to the melting point of silicon in an inert gas atmosphere, and temperature decreasing in the heat treatment is performed in an atmosphere containing 1-60% by volume of hydrogen.
Thus, by subjecting a silicon wafer to a heat treatment at temperature of from 1000xc2x0 C. to the melting point of silicon in an inert gas atmosphere, crystal defects of the wafer surface layer can be eliminated first. Then, by decreasing the temperature in an atmosphere containing 1-60% by volume of hydrogen during temperature decrease of the heat treatment, microroughness can be improved thanks to migration of silicon atoms on the wafer surface. In this method, since the amount of hydrogen gas used may be small, the safety of the heat treatment step can also be improved.
In the above method, the aforementioned inert gas atmosphere preferably consists of an argon atmosphere or an argon atmosphere containing 30% by volume or less of hydrogen.
This is because argon is easily handled, and even when it contains hydrogen, etching due to hydrogen contained in the atmosphere hardly occurs if its concentration is 30% by volume or less, and the effect of improving microroughness on the wafer surfaces will become higher to the contrary.
The present invention also provides a method for heat treatment of silicon wafers under a reducing atmosphere containing hydrogen by using a rapid heating and rapid cooling apparatus, wherein temperature decreasing rate from the maximum temperature in the heat treatment to 700xc2x0 C. is controlled to be 20xc2x0 C./sec or less.
By employing such a simple method as described above, i.e., only by using a temperature decreasing rate of 20xc2x0 C./sec or less from the maximum temperature in the heat treatment to 700xc2x0 C., in a method for heat treatment of silicon wafers using a rapid heating and rapid cooling apparatus, haze can be improved while COPs of the wafer surface are simultaneously reduced.
In the above method, it is preferred that the temperature decreasing rate in a region below 700xc2x0 C. in the heat treatment should be faster than the temperature decreasing rate from the maximum temperature to 700xc2x0 C.
By using a temperature decreasing rate in a region below 700xc2x0 C. in the heat treatment faster than the temperature decreasing rate in a region of from the maximum temperature to 700xc2x0 C. as described above, the whole heat treatment time can be shortened, and thus the efficiency of the heat treatment can further be improved.
In the above method, it is preferred that the aforementioned reducing atmosphere containing hydrogen should be 100% hydrogen or a mixed gas atmosphere of hydrogen with argon and/or nitrogen.
Such a heat treatment atmosphere can surely reduce COP density on the wafer surface and improve haze.
Moreover, in a silicon wafer subjected to the heat treatment by the aforementioned method of the present invention, COP density on the wafer surface is decreased and haze is made small by the simple method, and thus the electric characteristics of the silicon wafer such as oxide dielectric breakdown voltage and carrier mobility can be improved. Therefore, extremely useful silicon wafers of extremely high quality can be obtained with high productivity.
Specifically, for example, the silicon wafer can be a silicon wafer having a crystal defect density of 1.0xc3x97104 defects/cm3 or more in the wafer bulk portion, a crystal defect density of 1.0xc3x97104 defects/cm3 or less in the wafer surface layer of a depth of 0.5 xcexcm from the surface, a crystal defect density of 0.15 defects/cm2 or less on the wafer surface and surface roughness of 1.0 nm or less in terms of the P-V value.
The term xe2x80x9cwafer bulk portionxe2x80x9d used herein means a portion of wafer present at a depth exceeding 0.5 xcexcm from the wafer surface.
Thus, the silicon wafer of present invention can be a silicon wafer with a low crystal defect density of 1.0xc3x97104 defects/cm3 or less in the wafer surface layer of a depth of 0.5 xcexcm from the surface and less surface roughness of 1.0 nm or less in terms of the P-V value, even though it had a high crystal defect density during the growth of a silicon single crystal. In addition, since it contains crystal defects required for gettering of impurities such as heavy metals in the bulk portion, semiconductor devices showing superior oxide dielectric breakdown voltage characteristics and carrier mobility can be produced and the yield of the device production can be improved, if the devices are produced by using the silicon wafer of the present invention.
As explained above, according to the present invention, there can be obtained a silicon wafer having few crystal defects on the wafer surface, a large thickness of the defect-free wafer surface layer and less microroughness on the wafer surface by using a specifically defined optimum composition of the heat treatment atmosphere in a method of heat treatment of silicon wafers. Therefore, the yield of the device production can be improved. In addition, since it becomes possible to minimize the amount of hydrogen used by the method of the present invention, the safety of the heat treatment operation can be secured.
In addition, according to the present invention, a heat treatment having both effects for eliminating defects such as COPs on wafer surfaces and improving haze can be performed in an extremely simple manner by using an improved temperature decreasing rate of the heat treatment in a method for heat treatment of silicon wafers using a rapid heating and rapid cooling apparatus. Thus, it becomes possible to obtain silicon wafers of higher quality compared with conventional ones with lower cost in a simple manner.